So I've been playing for a while now, and I assume that there is the possibility of naval trade, since there are missions to reduce the risk on certain routes, but I don't know when naval trade is unlocked. Any clues?

I don't know when it is unlocked but it doesnt' telegraph it. I just happened to notice that I could build naval trade vessels (or something like that) in the crafting interface.
– ChrisNov 20 '12 at 19:20

2 Answers
2

About an hour after asking this, I found out. You need to do Lance the Woodworker's homestead mission where he goes to Boston for his tools. After that, the recipe will be unlocked and you can craft naval convoy. Then you can do naval trade. Hurrah.